Method for making concrete weather-resistant

ABSTRACT

The invention relates to a method of making concrete used in hydrotechnical structures weather-resistant as regards the freezing of water absorbed into the pores of the concrete. The problem in this connection consists of the expansion of freezing water and the consequent state of tension in the concrete. According to the invention, a small amount of micro-sized rubber powder is mixed with the concrete mix, the rubber particles forming even-surfaced, closed buffers into which the water expanding when it freezes can expand without producing states of tension in the concrete. The rubber particles are ground from the rubber parts of used automobile tires.

This application is a continuation of application Ser. No. 403,665,filed July 28, 1982, abandoned.

The present invention relates to a method of making concreteweather-resistant and particularly to a method in which allowance ismade for water absorbed into the pores of concrete to expand when thewater freezes.

In the concrete industry, the expansion of freezing water causesproblems especially in hydrotechnical structures. Water which isabsorbed into the pores of concrete and freezes there producesmicro-cracks, which in the course of time reduce the strength of theconcrete. Only recently has a realistic picture been obtained of thismicro-process, but satisfactory methods for preventing damage have notbeen known so far.

In order to prevent damage caused by freezing water it is necessary toproduce in the concrete, in one way or another, micro-pores which arenot filled with water absorbed into the pores of concrete but whichallow freezing water to expand into them. In other words, the pores mustbe made to serve as buffers.

According to one method so far used, air bubbles are formed in concrete.For this purpose, a surface-active agent is added to the concrete mix,and as a result the surface tension of the water decreases, whereby airbubbles are produced in the concrete.

This known method can be said to have the following disadvantages:

the pores are too large in proportion to the capillaries of concrete anddecrease the strength of the concrete

the entrained air does not remain homogeneously distributed, and thenumber of bubbles decreases during transport

the additional air-entraining agent used for the formation of thebubbles is in part absorbed during the preparation and transportingstage of the concrete mix by the carbon of fly-ash, which is nowadaysincreasingly used as a cement addition. Thus it has proven to be verydifficult to obtain the correct quantity of additional entrained air inconcrete.

Attempts have been made to reduce the said disadvantage by developingappropriate micro air entraining agents. One such agent is, for example,commercially available Kleenopor SA-8, which consists of plasticblisters. The diameter of the plastic blisters corresponds to thedimensions of the capillaries in concrete, and thus overdimensionedvoids will not be produced. This and similar micro air entraining agentshave, however, the following disadvantages:

the agent is very expensive

the plastic blisters are very lightweight, and special measures arerequired for mixing them with the concrete mix

the surface of the plastic blisters is weak, and so in order to avoidbreakage of the blisters, the mixing must be carried out in a preciselydetermined order

the film of the plastic blisters does not tolerate temperatures above+70° C., and therefore the blisters are not suitable for theincreasingly common hot-concrete projects, in which the temperature ofthe concrete mix is +80° C.

By the method according to the present invention it has been possible toeliminate the above-mentioned disadvantages.

In accordance with the invention, a suitable amount, determined throughexperiments, of a micro-sized rubber powder is mixed with the concretemix; advantageously the powder has been ground from used automobiletires. The ground rubber may be natural rubber or synthetic rubber or,preferably, mixtures of the same. An advantageous mixture consists of30% natural rubber and 70% synthetic rubber, for example in the form ofstyrene butadiene and polybutadiene.

Rubber powder is used precisely in the amount optimized according toneed, the amount being between 0.2 and 2% by weight of the amount ofcementing agent. The optimum particle size for the powder is between 10and 250 μm.

The mixing of a rubber powder prepared from used rubber, especially usedautomobile tires, with concrete mixes is previously known, but not forthe same purpose and not with the same results.

The object of Swiss Pat. No. CH-602,515 is to produce more lightweightconcrete by mixing with the concrete mix as a filler 10-90% plasticand/or waste rubber with macroscopic dimensions. The main objective isto achieve sufficient adherence between the filler and the othercomponents in order to maintain the strength of the concrete both duringcuring and in the final cured state.

The objective of German Pat. No. DE 1,011,803 is to produce lightweightgrouting concrete, especially for bases and floors. The formed concreteshould have improved thermal and sound insulation properties and itshould have better than previous water repellance. This is achieved bymixing as a filler with the concrete mix, rubber of macroscopicdimensions. By means of mineral additive fibers the stresses are evenedout so as to be three-dimensional.

The objective of British Pat. No. GB 1,362,614 is to provide alightweight plaster or concrete with improved thermal and soundinsulation properties. This is achieved by mixing with the mix as afiller, for example, rubber ground from used automobile tires, in aminimum amount of 50% and with a maximum particle size of 10 mm.

The objective of European Patent Publication No. 003469 is to compensatefor the shrinkage of concrete mixes. This objective is achieved bymixing, with the concrete mix, rubber ground from used automobile tires,the maximum particle size of the ground rubber being 10 μm and itsmaximum amount 35% of the reinforcement fiber amount. The item ofapplication thus also requires the use of reinforcement fibers, in whichcase the ratio of the amount of rubber to the amount of fiber isimportant.

However, none of these known methods deal with the problems due tofreezing of the water absorbed in the pores of concrete or the solvingof these problems, and only the method according to European PatentPublication No. 003469 uses micro-sized rubber powder, less than 10 μm,which is, however, smaller than in the present method, but in additionto rubber powder it uses reinforcement fibers and the object of themethod is to compensate for the normal shrinkage of concrete.

The method according to the present invention is thus based on the ideaof replacing the previously used entrained air and plastic blisters withmicro-size rubber powder particles. According to need, a preciselyoptimized amount, 0.2-2% of the amount of cementing agent, of groundrubber, especially rubber powder prepared from used automobile tires, ismixed with the concrete mix. The rubber used is thus in general amixture of natural and synthetic rubber and advantageously contains, forexample, 30% natural rubber and 70% synthetic rubber, such as styrenebutadiene and polybutadiene. By means of the rubber powder, spaces intowhich the pressure of water freezing in the concrete can be released arecreated in the concrete. When the ice melts, the compressed elasticrubber again forces the water out and is ready to compress again underpressure. The optimum particle size can be found within the range 10-250μm. In cured concrete the rubber particles form even-surfaced, closedmicro-pores. The even surface is important in order to preventdislocations.

The effective closing is important in order for the pores to serve asbuffers for the water which expands when it freezes.

The resistance of concrete to weather as regards freezing expansion isexpressed in concrete technology by means of the so-called freezingexpansion number. The freezing expansion expresses the change in lengthof a test specimen during the freezing stage as compared with the changein length calculated according to the temperature coefficient. The morenegative this number is, the better the resistance of concrete to thestresses due to freezing water. When the number is positive, a state oftensile stress prevails in the concrete test specimen, i.e. a state oftension in which concrete is susceptible to cracking.

In all test specimens prepared according to the invention and checked,the freezing expansion number was negative, and a couple of examples ofthis are given below:

EXAMPLE 1

Amount of rubber powder: 0.5% by weight of the cementing agent

Particle size: approx. 100 μm

Freezing expansion: -27 μm/m

EXAMPLE 2

Amount of rubber powder: 1.5% by weight of the cementing agent

Particle size: 80-200 μm

Freezing expansion: -42 μm/m

The substantial advantages of the method according to the invention areas follows:

A concrete mix with a rubber powder admixture naturally withstands thetemperature, 80° C., required by hot concrete. The method is easy tocarry out, since ground rubber flows readily and does not form intolumps and can therefore be batched directly into the concrete mix. Thepreparation of the rubber powder is also simple. The rubber part ofdiscarded automobile tires is peeled off by a known method and is groundto a suitable diameter size. The grinding can, when necessary, becarried out at a low temperature in order to minimize the grindingenergy requirement. One of the greatest advantages of the methodaccording to the invention is thus its simplicity and above all its lowprice, which is substantially affected by the fact that the raw materialused is waste material.

I claim:
 1. A weather-resistant, high-strength concrete for structuressubject to water that freezes to ice and melts again depending onambient temperature, comprising: a concrete mix, including a cementingagent and rubber powder mixed in the concrete mix, the particle size ofsaid rubber powder being between 80 and 250 μm and the amount thereofbeing 0.2-2% by weight of the cementing agent, whereby water absorbedinto pores of the concrete may expand upon freezing into space occupiedby the rubber powder particles and compress the same.
 2. The concreteaccording to claim 1, wherein said rubber powder consists of ground usedautomobile tires.
 3. The concrete according to claim 1 or 2, whereinsaid rubber powder is natural rubber.
 4. The concrete according to claim1 or 2, wherein said rubber powder is synthetic rubber.
 5. The concreteaccording to claim 1 or 2, wherein said rubber power is a mixture ofnatural and synthetic rubber.
 6. The concrete according to claim 5,wherein said rubber powder is a mixture of 30% natural rubber and of 70%synthetic rubber.
 7. The concrete according to claim 6, wherein saidsynthetic rubber is styrene butadiene.
 8. The concrete according toclaim 6, wherein said synthetic rubber is polybutadiene.